Update expected swift-testing failure message

[award999]

Description

Looks like the expected format of the main toolchain's failure message has changed

+ expected - actual

       "test": "PackageTests.MixedSwiftTestingSuite"
     }
     {
       "issues": [
-        "testFailing() › Expectation failed: 1 == 2\n1 == 2 → false"
+        "testFailing() › Expectation failed: 1 == 2"
       ]
       "test": "PackageTests.MixedSwiftTestingSuite/testFailing()"
     }
   ]

Issue: N/A

Tasks

• Required tests have been written
• [ ] Documentation has been updated
• [ ] Added an entry to CHANGELOG.md if applicable